Sometimes developers decide to build sites completly in Javascript that wants to have Flash look and feel. For me it’s a bad choice, mostly because of really bad experience, taking bad side in performance and experience because often there are no preloader no real fading no ease and soft transitions even they are trying to make it. It’s technically cool but that doesn’t makes me love the site.
On the other hand some Javascript developers push the limits making some 3d experiments or excellend 2d games in Javascript.
Tetris is well know game, possibly more then 2 decades passed after first version, and now I see it in Javascript, great implementaion and very playable, Tetris is not so processor demanding game. Great stuff, check it out (there is also link to the older version at the page)…
Cyberspace Nova 
Amazing